Polystyrene that has been polymerized with an amount of rubber, such as polybutadiene (PB), can be termed high impact polystyrene, or HIPS. The addition of polybutadiene increases the polymer's toughness and impact absorption.
The mechanical, optical and other properties of HIPS are greatly influenced by rubber particle size (RPS) and morphology. For instance, large rubber particles increase the toughness of HIPS, while small rubber particles increase hardness and gloss. Certain applications, such as cap layers for linings in refrigerators and other appliances, require HIPS with both high gloss and high impact strength.
Styrene butadiene (SB) copolymers can be combined with conventional HIPS formulations. For example, a portion of the polybutadiene used in HIPS synthesis can comprise diblock SB and/or triblock SBS rubbers, which are styrene butadiene copolymers.
SB copolymers, when included in a HIPS polymerization, tend to decrease RPS and lead to core-shell morphology, whereas polybutadiene rubber leads to larger RPS and cell or salami structure. Thus, altering the relative amounts and type of both PB and SB copolymers in a HIPS composition can affect the composition's RPS and morphology, and in turn affect important physical properties like impact absorption and gloss.
Prior art approaches generally have involved bimodal RPS distribution, in which a combination of small rubber particles and larger rubber particles impart to the HIPS both the characteristics of high gloss and high impact absorption. Different RPS distributions correspond to different morphologies. For instance, larger RPS, between 0.5 and 10 microns, generally shows a cellular or salami morphology, which is characterized by cells of rubber dispersed in the polystyrene matrix wherein each rubber cell has multiple occlusions of polystyrene either partly or completely trapped within the rubber cell. Smaller RPS, from about 0.1 to 0.5 micron, is usually characterized with capsule, core-shell, or dot morphology.
There exist a great variety of SB copolymers that vary in molecular weight, ratio of styrene to butadiene, and other features, all of which can influence a HIPS composition differently. Furthermore, varying kinds of PB rubber, altering amounts of PB and SB copolymers used in a HIPS composition, and adjusting amounts of other additives and reactions conditions all influence the physical properties of HIPS. Thus, an ongoing need exists for novel HIPS compositions that possess certain physical properties tailored to meet a specific end-use. The present invention is directed towards a novel HIPS composition and process for making the same that has both high gloss and high impact absorption.